Aircraft, as well as other vehicles, have primary power systems in the form of piston or gas turbine engines that provide, in addition to propulsion, shaft power to drive accessories such as electric generators and hydraulic pumps. These vehicles may also have a secondary power system as back up to the primary power system for when the aircraft is on the ground and the primary power system is shut down or during an inflight emergency.
APUs are air breathing gas turbine engines commonly used as secondary power systems. The APU provides shaft power to drive electrical generators, hydraulic pumps and other equipment requiring constant speed operation. The APU can also provide pressurized air for main engine starting, cabin air-conditioning and pressurization, de-icing, and air turbine motors. Historically, APU's have only been operated when the aircraft was on the ground. However, recent advances have extended the operating envelop of the APU so that it can now be started and operated at altitudes of about 45,000 ft. Nonetheless, a starter motor is required to accelerate the APU to its light off speed. With conventional starter motors, it takes about 30 seconds to a minute, depending on ambient conditions, before the APU can generate power. In addition, the APU is sensitive to aircraft attitude. For example, should the aircraft go into a spin the air entering the APU may be sufficiently distorted as to prevent the starting of the APU.
Aerodynamically unstable aircraft need a secondary power system that can provide power within 3-4 seconds at any altitude or aircraft attitude. Without hydraulic power to move control surfaces, or electrical power for flight control computers, these aircraft cannot be maintained in controlled flight. To meet these conditions, these aircraft have employed EPUs, instead of APUs, as secondary power systems.
An EPU is comprised of a combustor, a turbine wheel and a stored propellant either a monopropellant such as hydrazine, or bi-propellant such as jet fuel and air. When power is required, the propellant enters the combustor is reacted and then drives the turbine wheel which delivers shaft power to drive electrical generators, hydraulic pumps and other equipment requiring constant speed operation. The operation of the EPU is independent of ambient conditions and it can deliver power in 3-4 seconds. However, the EPU can only operate for the short period of time until the stored propellant is spent and therefore cannot perform many of the other functions required of a secondary power system such as main engine starting.
U.S. Pat. No. 4,864,812 discloses a combined auxiliary and emergency power unit. In an emergency, the EPU provides power until the APU can be started. However, an air turbine starter, hydraulic starter or electric starter is still required to start the APU. These starters not only add weight to the system but also decrease the system's reliability, maintainability and serviceability.
Accordingly, there is a need for a secondary power system that combines an APU with an EPU but does not require a separate starter for the APU.